How to define External Boundary for the Cylinder?

Hi all,

I am trying to initiate an SPIS-EP simulation 6.1.0. My Geometry is 1m radius and 2m long Cylinder. I have imported the STEP file in GMSH and defined the surfaces and volume. However, in the SPIs-Group editor, I am unable to define the external boundary for the cylinder. As shown in the figure, it is the cylinder surface (red) and not the external boundary. Do I need to create another simulation box surrounding the original geometry? If yes, then can I create in SolidWorks and then import in gm! Please help. Thanks in advance.

Cheers

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